Need help immediately: user lock-out

I have a client whos computer is used by approximately 8 users. The computer will automatically lock-out users using the last user that was logged on. The problem occurs after someone has logged off and leaves. This problem just started occuring and nothing was done to the computer before hand. I checked the services and I couldn’t find a solution there and i’m out of ideas on the solution. If somebody could provide any insight as to what the problem might be, then that would be greatly appreciated. Thank you.